A measuring cell for detecting hydrogen cyanide is disclosed in U.S. Pat. No. 4,227,974.
In the known measuring arrangement, an Ag/Ag.sup.+ measuring electrode is utilized at which however hydrogen cyanide (HCN) reacts with the Ag+-ions present in solution to form a complex formation as represented by equations (1) and (2) below: EQU HCN=H.sup.+ +CN.sup.- (1) EQU Ag.sup.+ +2CN.sup.- =Ag(CN).sub.2.sup.- (2) EQU Ag=Ag.sup.+ +e.sup.-1 (3)
In this way, the electrochemical equilibrium is disturbed and the measuring electrode delivers Ag.sup.+ -ions while partially decomposing until the equilibrium is restored as shown in equation (3). This condition produces electrode corrosion over a long term which influences the long-term stability of the measuring signal and leads to an undesired drift in the response of the measuring cell.
In view of the above, hydrogen cyanide is detected in known measuring cells in that the formed cyanide forms a complex with the dissolved silver ions thereby disturbing the electrode equilibrium so that the electrode must resupply Ag and consequently corrodes.